The present invention relates to a novel fluid catalytic cracking catalyst comprising a catalytically active first component and a second component for reducing the emissions of oxides of sulfur from the regenerator of a fluid catalytic cracking unit and the use of that catalyst to crack sulfur containing petroleum feedstocks.
Modern fluid catalytic cracking units comprise a cracking zone which is sometimes referred to as the reactor and a regeneration zone which is sometimes referred to as the regenerator. In the cracking zone, a petroleum feedstock is contacted with a fluid catalytic cracking catalyst comprising Y-faujasite at a temperature of about 800.degree.-1100.degree. F. to crack the hydrocarbon molecules in the feedstock and to produce a complex mixture of vaporous products, including products which are sometimes referred to as dry gases (hydrogen, hydrogen sulfide, and hydrocarbon gases containing one or two carbon atoms), gases (hydrocarbon gases containing three or four carbon atoms) and gasoline (hydrocarbons having a boiling point under about 420.degree. F. and containing five or more carbon atoms).
During cracking, a carbonaceous deposit called coke is deposited on the cracking catalyst. As will be described in more detail below, after the cracking catalyst containing coke is separated from the vaporous products of the cracking reaction, the coke is removed from the cracking catalyst in the regeneration zone. The regenerated catalyst is then recirculated to the cracking zone where it cracks additional petroleum feedstock.
The petroleum feedstock that is cracked in the cracking zone typically contains sulfur which is chemically combined with the hydrocarbon molecules in the feedstock. During cracking, most of the sulfur in the feedstock is converted to a vaporous form in which it is removed from the reactor along with the other vaporous products of the cracking reaction. Generally, about 40-60% of the sulfur in those vaporous products is in the form of hydrogen sulfide. The hydrogen sulfide may be recovered from the other vaporous products by methods well known in the art.
Not all the sulfur in the petroleum feedstock is removed from the reactor with the vaporous products of the cracking reaction. In particular, a minor portion of the sulfur in the petroleum feedstock (e.g., 5-10% by weight of the sulfur in the feedstock) is deposited, along with the coke, on the cracking catalyst itself.
The cracking catalyst containing coke and sulfur is regenerated in the regeneration zone by contacting it with air at high temperature, e.g., 1200.degree.-1600.degree. F., to burn the coke off the catalyst and to produce a flue gas containing oxides of sulfur, oxides of carbon and particulate materials. Such flue gases sometimes contain 2000 parts per million by volume (ppmv) or more of oxides of sulfur.
Particularly during the past 10 years or so, a substantial amount of effort has been devoted to the study and development of techniques for reducing the quantity of the oxides of sulfur in flue gases emitted from fluid catalytic cracking regenerators. For example, we understand that in the early 1970's the Environmental Protection Agency contracted the Monsanto Research Corporation to identify conceptual techniques for reducing the quantity of the emissions of oxides of sulfur from fluid catalytic cracking regenerators and to perform a feasibility analysis of the techniques identified. A report on the first phase of that study describes a survey of over 100 conceptual techniques which were said to be "applicable to fluid catalytic cracker (FCC) regenerator off-gas sulfur dioxide emission reduction". See Ctvrtnicek, Refinery Catalytic Cracker Regenerator SO.sub.x Control Process Survey, EPA Report No. 650/2-74-082, at Abstract and pp. 70-83 (September 1974).
One approach for reducing the quantity of the oxides of sulfur in regenerator flue gases that has received considerable attention is the addition of a metallic reactant such as a metallic oxide to the circulating cracking catalyst Bertolacini U.S. Pat. No. 4,376,103 (the '103 patent) reports at col. 3, line 58-col. 4, line 20 that the metallic oxide "absorbs the sulfur oxides produced in the regenerator" by the following chemical reactions, which are said to be reversible: EQU M.sub.x O+SO.sub.2 .fwdarw.M.sub.x SO.sub.3 EQU M.sub.x O+SO.sub.3 .fwdarw.M.sub.x SO.sub.4 EQU M.sub.x O+SO.sub.2 +1/2O.sub.2 .fwdarw.M.sub.x SO.sub.4
where M is the metal and x is the ratio of the oxidation state of the oxide ion to the oxidation state of the metal ion.
The '103 patent discloses that the sulfites and sulfates that are "absorbed" by the metallic oxide are liberated as hydrogen sulfide gas in the reducing atmosphere of the cracking zone of the cracking unit by the following reactions: EQU M.sub.x SO.sub.3 +3H.sub.2 .fwdarw.M.sub.x O+H.sub.2 S+2H.sub.2 O EQU M.sub.x SO.sub.4 +4H.sub.2 .fwdarw.M.sub.x O+H.sub.2 S+3H.sub.2 O EQU M.sub.x SO.sub.3 +3H.sub.2 .fwdarw.M.sub.x S+3H.sub.2 O.fwdarw.M.sub.x O+H.sub.2 S+2H.sub.2 O EQU M.sub.x SO.sub.4 +4H.sub.2 .fwdarw.M.sub.x S+4H.sub.2 O.fwdarw.M.sub.x O+H.sub.2 S+3H.sub.2 O
where M and x are as above.
As can be seen from the above equations, in connection with the liberation of hydrogen sulfide gas in the cracking zone, the metallic material is converted back to its metallic oxide form so that it can "absorb" additional oxides of sulfur when it is recirculated to the regeneration zone.
In theory, the use of a metallic reactant such as a metallic oxide to reduce the quantity of oxides of sulfur in regenerator flue gases has a number of advantages. Some of those advantages are described in the '103 patent at col. 3, line 63-col. 4, line 5 as follows:
"This approach is so attractive because the sulfur thus shifted from the regenerator flue gas to the reactor effluent is simply a small addition to the large amount of hydrogen sulfide invariably present in the reactor effluent. The small added expense, if any, of removing even as much as 5 to 15 percent more hydrogen sulfide from an FCC reactor gas stream by available means is substantially less than, for example, the expense of separate feed desulfurization or flue gas scrubbing to reduce the level of sulfur oxides in the regenerator flue gas."
Despite these advantages, there are a number of obstacles which togehter have inhibited the development of a commercially practicable catalyst that includes a metallic reactant for substantially reducing the quantity of oxides of sulfur in regenerator flue gases. One obstacle is the requirement that the metallic reactant be capable of substantially reducing the quantity of oxides of sulfur in the flue gas under the conditions present in the regeneration zone and at the same time be capable of releasing a substantial amount of the "absorbed" sulfur as hydrogen sulfide gas under the conditions present in the cracking zone.
Another obstacle is that the metallic reactant must be capable of withstanding the conditions in the catalytic cracking unit. In particular, the metallic reactant must be in a form that is sufficiently resistant to attrition and to other adverse effects in the cracking unit (e.g., deactivation of the metallic reactant by silica migration) to be commercially practicable.
The metallic reactant must also not detract from the characteristics of the other components of the catalytic cracking catalyst in a commercially unacceptable way. In this connection, it has long been recognized that for a fluid catalytic cracking catalyst to be commercially successful, it must have commercially acceptable activity, selectivity, and stability characteristics. It must be sufficiently active to give economically attractive yields, it must have good selectivity towards producing products that are desired and not producing products that are not desired, and it must be sufficiently hydrothermally stable and attrition resistant to have a commercially useful life.
One product that is undesirable in commercial catalytic cracking processes is excessive coke. Even small increases in the yield of that product relative to the yield of gasoline can cause significant practical problems. For example, such increases can cause undesirable increases in the heat that is generated by burning off the coke during the highly exothermic regeneration of the catalyst.
Other products that are undesirable are dry gases and gases, particularly the dry gases. One reason for this is that in commercial refineries expensive compressors are used to handle dry gases and gases. Increases in the volume of those products produced, therefore, can add substantially to the capital expense of a refinery.
Because of all the foregoing constraints, it has proven difficult to develop commercially practicable fluid catalytic cracking catalysts that include a metallic reactant for substantially reducing the quantity of the oxides of sulfur present in regenerator flue gases.